Multiple sclerosis (MS) is the most common inflammatory demyelinating condition of the CNS, and affects about 400,000 people in the United States. In most patients the condition begins with a relapsing/remitting course, but then evolves to a progressive phase in which earlier successful therapies are no longer effective for treating disease. The most consistent biochemical marker for MS is increased antibody in the CNS, antibody that is produced from cells within the nervous system and is more clonally restricted than the antibody found in the blood. Yet the targets of the antibody and the specific immunopathogenic mechanisms in MS are not known. Previous studies have identified repertoires of antibody sequences expressed in the demyelinating lesions, and developed methods to synthesize functional recombinant antibodies that replicate the in vivo functions. This application will examine the specificity of the intrathecal antibodies produced by resident B and plasma cells in the lesions of relapsing/remitting MS brains, using state-of-the-art immunologic techniques that include staining, precipitation and immunopanning techniques developed in this lab. The proposal will also microdissect individual B and plasma cells from postmortem lesions in secondary progressive MS brains to examine their antibodies and targets. These parallel studies may uncover differences in the antibody profiles produced in progressive MS patients compared to relapsing remitting disease, or shifts in the targets or mechanisms by which the progressive course of disease is refractory to B cell therapies. This proposal always examines the antibody that is expressed in the affected brain lesions of MS patients, and is most likely to provide clues to the disease process. The pathogenic B cell response in MS may operate by antibody- independent mechanisms such as production of pro-inflammatory cytokines or activation of harmful T cells, but activation of the pathogenic B cells requires antibodies on their surfaces to bind to their targets. This proposal uses innovative research strategies to discover the underlying nature of the immune response in MS, the fundamental targets of antibody that is expressed from first diagnosis throughout the worsening of disease. Knowledge of these targets would provide new strategies for therapies, and potential biomarkers for diagnosis and prevention of disease.